Wobble dies find employment in the shaping or finishing of workpieces, in particular with such workpieces that have a non-rotationally symmetric geometry and/or undercuts. In German patent DE 196 37 839 there is described for example the working of the ball raceways and cage guide surfaces of an outer part of a Rzeppa constant velocity joint by means of a wobble die. With the method there described, initially a wobble die, segment-wise differently convexly curved, is introduced into a hollow chamber of a preliminary workpiece. Due to different curvatures of the segments, the wobble die has rib-like contours on its outer surface, which find employment for working the ball raceways. Then, the wobble die is set into a wobble movement whereby it rolls over the surfaces of the preliminary workpiece which are to be shaped in a plurality of circular orbits.
With the methods described in DE 196 44 639 of a workpiece is worked. Here there is involved a ratchet wheel and coaxially arranged and axially projecting ratchet teeth which have an axial undercut. Here also, the wobble die and the preliminary workpiece are set into a wobbling circular motion relative to the contours of the wobble die move over the outside of the preliminary workpiece shaping manner.
For the wobble movement, various kinds of movement are conceivable which will be briefly explained below with reference to FIGS. 1 and 2. During the placing or removal of the wobble die, the preliminary workpiece and the wobble die are oriented in common along a main axis 1.
During the movement, the wobble axis 2—if the wobble die alone is set into motion, this corresponds to the middle axis of the die—is pivoted out with respect to the central axis by the wobble angle α. The wobble axis 2 always goes through the wobble point 3, that is, this wobble point 3 is the neutral centre of the wobble movement. The different kinds of movement are illustrated by means of a point on the wobble axis 2 which point moves with this axis. Four different kinds of movement are illustrated in FIGS. 2a to 2d. The circular movement (FIG. 2a) is suitable for example for the working of annular undercuttings. With the linear movement (FIG. 2b) a pendulum motion is carried out, whereby the angle β, which is enclosed by a predetermined plane of the wobble press and the pendulum plane, can be set to any desired value. A working of the workpiece at regular spacings is attained by means of the loop-like movement of FIG. 2c. Finally, FIG. 2d shows a spiral-shape wobble movement, with which the pressing force of the die can be continuously reduced or increased.
For the generation of such wobble movements there are suitable many kinds of wobble presses, which are classified into several types in accordance with their die movements.
With the wobble press of type 1, a lower die with the workpiece placed therein carries out a driven rotational movement around the machine axis. The wobble press die rotates therewith around the wobble axis inclined by the wobble angle α, synchronously to the rotational movement of the lower die. A translational movement, which corresponds to a stroke movement during the forming process, is likewise introduced by means of the wobble press die press die hydraulically. This translational movement is carried out in order to hold the wobble press die 4 at each point in time at a suitable height with respect to the workpiece 1. If the rotational movement of the wobble press die is not driven, i.e. the die rotates freely around the wobble axis together with the workpiece, one speaks of the wobble press type 1A. If, in contrast, the rotational movement of the wobble press die is additionally driven, this is called a wobble press type 1B.
The wobble press of type 2 has a fixed lower die which can carry out neither translational nor rotational movements. The workpiece is stationary during the entire working process. In this case the wobble press die carries out three movements overall, a rotation around the wobble axis, a wobble movement around the machine axis, and the hydraulically driven stroke movement in the axial direction.
Type 3 common wobble presses represent, in contrast, the most kind of wobble press. Here, the hydraulically stroke movement is effected by means of a cylinder in the lower part of the machine. The axial translational movement is thus carried out by means of the outer die. The wobble movement is introduced by means of the wobble press die, lower die and workpiece remaining stationary with regard to the rotational movement.
In general it is to be noted that the wobble point—considered mathematically—may also be infinitely distant, which leads to a rolling movement. This rolling movement thus represents an extreme case of a wobble movement.
The shape of the wobble die is dependent on the one hand on the desired contour of the preliminary workpiece. Thereby it must be ensured that the wobble die can be put and again removed without problem, before and after the wobble process. For the working of the bellshaped outer part of a Rzeppa constant velocity joint this means, for example, that the maximum cross-section of the wobble die may not be greater than the opening. This problem arises in general whenever undercuttings are to be worked with the wobble die. Further, in the configuration of the wobble die, the wobble movement must be taken into account, whereby not every wobble movement is suitable for working a desired structure. For example, the wobble angle α is dependent also upon the depth of the ball raceways which are to be shaped. Further, during the wobble process, elastic deformations may appear on the workpiece and die, so that the actually attained result deviates from the theoretically calculated.